The M16 rifle, and its civilian counterpart the AR-15, was originally developed by American engineer Eugene Stoner of ArmaLite Inc. in the late 1950s. The rifle was notable for its light weight, its accuracy, and its relative capacity to fire large amounts of ammunition. The Stoner auto loading design was the subject of U.S. Pat. No. 2,951,424, which issued to E. M. Stoner on Sep. 6, 1960. Specifically, the '424 patent discloses the M16 bolt and bolt carrier system and the gas operation thereof. The system utilizes a gas tube that extends from a gas port in the barrel, back into the upper receiver of the rifle and into a gas tube pocket or “key” attached to the bolt carrier. The original Stoner design is frequently referred to as a “gas impingement” or “direct impingement” system.
Direct impingement or gas impingement is a type of gas operation for a firearm designed to expel a spent cartridge and load a new cartridge using the gas that is discharged from a cartridge as it trails a bullet down the barrel of a rifle. In a gas impingement system, the gas from firing a cartridge is directed down the barrel of a rifle and enters a gas tube at or toward the distal end of the barrel. The gas tube forms a conduit through which the gas is propelled back to the bolt carrier or slide assembly to cycle the action in the firearm. More specifically, in a direct gas impingement system, when the firearm is fired, the exhaust propellant gases from the fired cartridge are directed through a port at the end of the barrel and then channeled back to the bolt carrier and will strike, or impinge, the bolt carrier moving it rearward toward the buttstock and into a retracted position. The exhaust gases will then discharge out the exhaust on the bolt carrier group or the ejection port on the side of the firearm near the buttstock. After discharge, the buffer tube spring, acting on the bolt carrier, will move the bolt carrier back to the engaged position, while simultaneously stripping and picking up another cartridge from the magazine and moving that cartridge into a battery position within the firearm's breech. Examples of direct gas impingement firearms include the AR-15, M4, AR-10, and M16 style firearms.
The conventional AR-15/M-16 gas-operated direct impingement system has been observed to have a number of short-comings. The principal shortcoming of this system is the deposits of residues that accumulate in the bolt from discharge gasses. The deposits decrease the reliability and usability of the rifle. Deposits inhibit the proper operation of the firearm, requiring frequent cleaning of the gas operating system. The discharge of gases into the bolt also creates excessive heat in the bolt and break down lubrication that normally enables the smooth operation of the bolt assembly. Additionally, performing the cleaning of the rifle bolt assembly under field conditions is difficult and requires specialized tools, which may not be available.
The original direct impingement system has been modified or replaced with a gas piston system in an effort to overcome some of the aforementioned shortcomings. Many of the designs are retrofit systems that entirely replace the gas tube with a piston and cylinder. In these systems, the piston head and cylinder are mounted to the gas block towards the distal end of the barrel. Discharge gasses flow out of the barrel and into the piston chamber where the gasses force the piston back towards the bolt carrier, driving the bolt carrier back toward the buttstock and into a retracted position. The exhaust gases can then discharge out from the firearm near the gas block. Like the gas impingement system, after discharge, the buffer tube spring acting on the bolt carrier will move the bolt carrier back to the engaged position, while simultaneously stripping or picking up another cartridge from the magazine and moving that cartridge into a battery position within the firearm's breech.
Because the gasses vent out of the firearm near the barrel, firearms using a gas piston system do not deposit as much residue in the bolt carrier assembly. They also do not heat up as much around the bolt carrier assembly. This enables rifles using a gas piston system to require less frequent maintenance. They also operate cooler in situations where large amounts of ammunition is fired over a short period of time. However, gas piston systems have their own drawbacks. First, rifles using gas piston systems are heavier than otherwise identical rifles using direct impingement systems. To compound the problem, the additional weight of the gas piston system is localized towards the barrel of the rifle, which can impair the maneuverability of the rifle in field situations. Second, rifles with gas piston systems have proven to be less accurate than otherwise identical direct impingement rifles. This may occur because the gas piston system reduces or eliminates the ability of the barrel to float, leading to diminished harmonics. Lastly, many gas piston systems have designs that render them difficult, or impossible, to service in the field. While these rifles generally require less service, they still must be serviced occasionally. Malfunctions also need to be addressed or cleared when they arise with the piston system. These malfunctions can occur due to the deposition of residues that will contact the piston system.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the previous direct impingement and gas piston systems. However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the field of this invention how the shortcomings of the prior art could be overcome.